3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels. As illustrated in FIG. 1, LTE transmissions are sent from base stations 102,104, such as Node Bs (NBs) and evolved Node Bs (eNBs) in a telecommunication network 100, to mobile stations 108, 110, 112 (e.g., user equipment (UEs)). Examples of wireless UE communication devices include mobile telephones, personal digital assistants, electronic readers, portable electronic tablets, personal computers, and laptop computers. The UEs operate within serving cells 116, 118 corresponding to nodes 102, 104, respectively. The communication network 100 may include one or more connecting networks 106.
The LTE standard is primarily based on Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, which splits the signal into multiple parallel sub-carriers in frequency. A transmit time interval (TTI) is the basic logical unit, which is comprised of a pair of sub-frames (or resource blocks). A radio resource element (RE) is the smallest addressable location within a TTI, corresponding to a certain time location and a certain frequency location. For instance, as illustrated in FIG. 2, a sub-frame 200 comprised of REs 202, 204 may be transmitted in a TTI in accordance with the LTE standard, and may consist of sub-carriers 206 in the frequency domain. In the time domain, the sub-frame may be divided into a number of OFDM (or Single Carrier Frequency Domain Multiple Access (SC-FDMA)) symbols 208. Thus, the unit of one sub-carrier and one symbol is a resource unit or element 202, 204.
Certain wireless communication systems, such as the system 100 shown in FIG. 1, may also include additional lower-power nodes or points 114, 124 within macro cells 116, 118. This may be referred to as a “heterogeneous” or “multi-layered” network deployment, wherein a mixture of nodes with different downlink transmission power, operate on (at least partially) the same set of frequencies and with overlapping geographic coverage. In a heterogeneous deployment, the low power nodes may not provide full coverage, but rather, may be deployed to improve capacity and data rates within their limited coverage areas, such as coverage area 120 and 122 of nodes 114 and 124 respectively. Node 114 and coverage area 120 may be a pico site of macro-cell 116. Similarly, node 124 may be a femto node, that may be employed for instance in a home, office building, or other structure. In some instances, node 124 may be associated with a Closed Subscriber Group (CSG) and limit access to members of the CSG.
A recent development with respect to heterogeneous deployments is the concept of “soft cell” (or “shared cell,” or “phantom cell”) schemes. In a soft cell deployment, an operator controls macro base stations and low power nodes in the same geographic area 116 such that control signaling is transmitted to users in the area via the macro base station 102 and data is transmitted via one or more lower power nodes, such as pico base station 114, for users 110 located in the coverage area 120 of the lower power node. In a soft cell configuration, the lower power nodes remain a part of the macro cell 116 rather than creating independent cells with, for instance, unique control signaling.
In order to maintain the highest possible quality of service, UEs in an LTE deployment periodically monitor not only the link quality to their serving cell, but also the link quality to neighboring cells. For instance, in the example network 100 of FIG. 1, UE 110 may consider both the quality of a link to base station 102 as wells as base station 104. If the serving cell 116 transmission quality is insufficient (e.g., certain metrics fail to meet required threshold levels), a handover to neighboring cell 118 may be initiated. A handover procedure may be categorized based on packet loss, for instance, it may be labeled “seamless” if it minimizes interference time, or “lossless” if the procedure does not tolerate any loss of packets.
As specified in 3GPP 36.331, version 10.5.0, handover may be executed based on the following comparison:RSRPTarget>RSRPServing+HOHysteresis+CellOffset  (1)where the terms RSRPTarget and RSRPServing refer to the Reference Symbol Received Power (RSRP) measurements from the target cell 118 and serving cell 116, respectively, in the network 100 of FIG. 1. The RSRP measurements in Equation (1) are determined from reference symbols transmitted from base stations 102, 104 and reported by the UE back to the serving base station. This equation has to be satisfied during a given period Time to Trigger (TTT) in order for handover from the serving cell to the target cell to be executed. Values for the parameters CellOffset, margin hysteresis HOHysteresis, and TTT are set to control the ease/difficulty of handoff to or from a given cell and are typically the same for all users within a given cell. However, the LTE standard allows for independently setting handover trigger parameters on a per-UE basis. Exemplary values for a network in a large urban European area are 1-3 dB for HOHysteresis and 320-960 ms for TTT. Equation (1) corresponds to the event A3 specified in §5.5.4 of 36.331, version 10.5.0., and is the criterion largely used in existing systems.
In a soft cell deployment, a UE 110 is formally associated with the macro cell 116, and thus, handover decisions are done on the basis of signals received by the UE 110 from the macro base stations 102, 104. In other words, if the existing handover procedures are applied to a “soft cell,” only measurements on the macro layer are taken into account for handover decisions.
Therefore, despite the existence of current protocols related to handover decision mechanisms, there remains a need for devices and methods that can address the potential discrepancy between the link quality on the control plane and the user (or data) plane. This discrepancy frequently arises in a heterogeneous network featuring the soft cell concept. When the quality experienced at the data plane (i.e., the signal received from the pico base station) is higher than the quality received at the control plane (i.e., the signal received from the macro base station), existing techniques will fail to make the optimal handover decision.